1. Field of the Invention
This invention relates to the gasification of coal and similar carbonaceous materials and is particularly concerned with a catalytic gasification process for producing an intermediate Btu gas carried out in the presence of a carbon-alkali metal catalyst.
2. Description of the Prior Art
Existing and proposed processes for the manufacture of synthetic gaseous fuels from coal or similar carbonaceous materials normally require the reaction of carbon with steam, alone or in combination with oxygen, at temperatures between about 1200.degree. F. and about 2500.degree. F. to produce a gas which may contain some methane but consists primarily of hydrogen and carbon monoxide. This gas can be used directly as a synthesis gas or a fuel gas with little added processing or can be reacted with additional steam to increase the hydrogen-to-carbon monoxide ratio and then fed to a catalytic methanation unit for reaction with carbon monoxide and hydrogen to produce methane. It has been shown that processes of this type can be improved by carrying out the initial gasification step in the presence of a catalyst containing an alkali metal constituent. The alkali metal constituent accelerates the steam-carbon gasification reaction and thus permits the generation of synthesis gas at somewhat lower temperatures than would otherwise be required. Processes of this type are costly because of the large quantities of heat that must be supplied to sustain the highly endothermic steam-carbon reaction. One way of supplying this heat is to inject oxygen directly into the gasifier and burn a portion of the carbon in the feed material being gasified. This method is highly expensive in that it requires the existence of a plant to manufacture the oxygen. Other methods for supplying the heat have been suggested but these, like that of injecting oxygen, are expensive.
It has been recently found that difficulties associated with processes of the type described above can largely be avoided by carrying out the reaction of steam with carbon in the presence of a carbon-alkali metal catalyst and substantially equilibrium quantities of added hydrogen and carbon monoxide. Laboratory work and pilot plant tests have shown that catalysts produced by the reaction of carbon and alkali metal compounds such as potassium carbonate to form carbon-alkali metal compounds or complexes will, under the proper reaction conditions, equilibrate the gas phase reactions occurring during gasification to produce additional methane and at the same time supply substantial amounts of additional exothermic heat within the gasifier. This additional exothermic heat of reaction essentially balances the overall endothermicity of the reactions involving solid carbon and the water-gas shift reaction and thus results in a substantially thermoneutral process in which the injection of large amounts of oxygen or the use of other expensive methods of supplying heat are eliminated.
The catalytic effect of carbon-alkali metal catalysts on the gas phase reactions, as distinguished from the solid-gas reactions or the reactions of carbon with steam, hydrogen or carbon dioxide, allows the following exothermic reactions to contribute substantially to the presence of methane in the effluent gas and drastically reduces the endothermicity of the overall reaction: EQU 2CO+2H.sub.2 .fwdarw.CO.sub.2 +CH.sub.4 (exothermic) (1) EQU CO+3H.sub.2 .fwdarw.H.sub.2 O+CH.sub.4 (exothermic) (2) EQU CO.sub.2 +4H.sub.2 O.fwdarw.2H.sub.2 O+CH.sub.4 (exothermic) (3)
Under the proper operating conditions, these reactions can be made to take place within the gasification zone and supply large amounts of methane and additional exothermic heat which would otherwise have to be supplied by the injection of oxygen or other means. Laboratory and pilot plant tests have shown that constitutents of the raw product gas thus produced are present in equilibrium concentrations at reaction conditions and consist primarily of hydrogen, carbon monoxide, carbon dioxide, methane and steam. It has been proposed to utilize steam gasification in the presence of a carbon-alkali metal catalyst to produce a high Btu product gas by treating the raw product gas for the removal of steam and acid gases, principally carbon dioxide and hydrogen sulfide; cryogenically separating carbon monoxide and hydrogen in amounts equivalent to their equilibrium concentration in the raw product gas from the methane in the treated gas; withdrawing methane as a high Btu product gas; and recycling the carbon monoxide and hydrogen to the gasifier. The presence in the gasifier of the carbon-alkali metal catalyst and equilibrium quantities of recycle carbon monoxide and hydrogen, which tend to suppress reactions that would otherwise produce additional hydrogen and carbon dioxide, results in a substantially thermoneutral reaction to produce essentially methane and carbon dioxide. Since the overall reaction is substantially thermoneutral, only a small heat input is required to preheat the carbonaceous feed material and to maintain the reactants at reaction temperatures by compensating for heat losses from the gasifier. This small amount of heat may be supplied by preheating the gaseous reactants in a conventional preheat furnace.
Although the above-described process permits the production of a high Btu product gas from a carbonaceous feed material without the necessity for separate downstream shifting and methanation steps and thereby significantly reduces the cost of preparing pipeline quality gas from coal and other carbonaceous materials, it does require the use of the relatively expensive steps of cryogenic separation and preheating, both of which result in a high comsumption of energy. Therefore, in situations where the production of a high Btu, pipeline quality gas is not necessary, it may be desirable to utilize a process for the thermoneutral gasification of a carbonaceous material in the presence of a carbon-alkali metal catalyst and added hydrogen and carbon monoxide that does not utilize a cryogenic separation and a preheat step.